High initial yield begins in stencil printing

Wertheim, 04.06.2020

As a system supplier, Ersa also covers the entire spectrum of electronics production in its webinar series – after “Selective mini-wave soldering”, the next English webinar for soldering systems dealt with the topic “Defects in the printing process and their consequences in the SMT line”. In the fully booked 60-minute web lecture Ersa Product Manager Wolfram Hübsch illuminated stencil printing from all sides.

The aim of the printing process in electronics production is to reduce errors to a minimum right at the beginning of the process – ideally: the zero error rate – and thus also to keep costs within an economic framework. In real production, it is not uncommon for minor problems to accumulate in the course of the individual process steps and turn into major difficulties. In addition to the printing and reflow process, there are therefore numerous influencing variables to be considered that can cause qualitative problems – such as the PCB, components, process, equipment used, the environment and also the operator. For a high first pass yield (FPY), a sophisticated strategy is needed that balances productivity and quality, but also keeps an eye on costs.

Above all, it is worthwhile to take a closer look at the influencing variables of the printing process, as the error potential in the SMT process can account for almost two thirds of the total –here too, a zoom in on equipment (process times/capability, tolerances), process (doctor blade parameters, cleaning, inspection), material (PCB, solder paste, stencil), environment (ESD, operator) and operator reveals possible improvements. In contrast to 20 years ago, stencil printing now has to deal with a wide range of components that have different requirements, for example, in terms of solder quantity and layout. A 2D partial inspection has long since ceased to be sufficient, and full-surface 3D inspection is now considered standard – especially for determining the volume of small, very fine pads.

So what does the perfect stencil print look like? It should be exactly shaped and print sharp-edged, flat solder depots with constant volume – a surface printed up to 90% is considered “perfect”, up to 70% “sufficient” – with less than half the surface the result is “unacceptable”. The aim of the SMT printing process is to achieve a sufficient volume for solder connections without solder bridges and with a constant solder volume after reflow soldering. The limit values for solder paste printing are determined by the printed solder area, bridging (not higher than 20%), volume and height.

“Ersa carries out a machine capability test on all machines before leaving the factory – this means that at least 50 PCBs are measured and the repeatability is determined with two external cameras,” explains Wolfram Hübsch. This ensures that the process limits of the machines are maintained. Printing errors can occur in the form of paste carry-over, print misalignment, solder bridges, soldering errors and as too high or too low a paste volume. For example, the printing offset can be due to a shrunken or stretched PCB or an inadequate calibration. Such an error can lead to bridging after the reflow process, but can also be caused by excessive paste application or placement offset. In addition to tips for sensible cleaning during stencil underside cleaning, there were also tried-and-tested tips on how to avoid gas inclusions called “gravestones”, solder beads or “voids” in QFN components.

The Ersa experts will be happy to answer individual questions on stencil printing and provide individual support to customers in optimizing their electronics production with future-proof, modular systems close to the zero-defect ideal.